Usually, sugar is obtained from beets by first cleaning the harvested beets, wherein they are cleaned from a large part of the still adhering soil and leaf residues. After passing a washing process, the beets are planed in cutting machines into pencil-sized slices. From the slices, sugar is produced through counterflow extraction using hot, slightly acidified water. By acidifying the extraction liquid, filtration of the raw sugar beet juice and the pressing properties of the extracted slices are improved. The raw sugar beet juice obtained during the extraction is subsequently fed to an extract purification process. Usually, the extract purification takes place with the aid of the so-called lime-and-carbonic-acid extract purification in the form of a pre-liming and a main liming process and also a first and second carbonation and the separation of the precipitation after the first and second carbonatations. The purpose of the extract purification is to largely remove the non-sucrose substances, in particular high-molecular substances, contained in the sugar beet juice. The non-sucrose substances to be removed should not be degraded here, if possible, so that no additional low-molecular substances get into the extract or raw sugar beet juice.
During pre-liming, the raw sugar beet juice is gradually alkalized under gentle conditions by adding lime milk. Pre-liming is carried out by adding defined amounts of calcium hydroxide (lime milk). Due to the alkaline treatment of the raw sugar beet treatment, neutralization of the organic and inorganic acids present in the extract as well as precipitation reactions of the anions, which form with calcium insoluble or hardly soluble salts, take place. Thus, for example, phosphate, oxalate, citrate and sulfate are precipitated to the greatest possible extent. Moreover, colloidally dissolved non-sucrose substances coagulate and are precipitated. The precipitation of individual ingredients, for example, of anions such as oxalate, phosphate, citrate, sulfate or of colloids such as pectin and proteins, takes place within certain pH ranges. Within these pH ranges, densification of the precipitation takes place at the same time. By adding lime milk during pre-liming, coagulation of protein takes also place. Due to this protein content, the aforementioned non-sucrose substances are also designated as protein-containing fraction from raw sugar beet juice.